In an image display device having fixed pixels, since the most popular liquid crystal display at present needs a backlight, a light emission amount of the backlight has to be increased to obtain higher luminance of a displayed image. However, when the light emission amount of the backlight is increased, although the luminance of the displayed image becomes high, the contrast becomes poor because the light cannot be completely blocked by the liquid crystal. Namely, luminance and contrast of a displayed image are in a trade-off relationship in a liquid crystal display, so that it is difficult to balance both at high levels.
As an image display device for eliminating the problem, there is known an image display device having self-luminous type pixels wherein a light emitting element is provided and a light emission amount thereof determines the luminance.
As an image display device having self-luminous pixels, for example, an organic EL display having elements using electroluminescence of an organic material is known. In the organic EL display, there are advantages that high luminescence is obtained with a relatively low voltage, there is not a viewing angle dependency, contrast is high and, furthermore, excellent display performance for motion pictures is obtained due to its good response.
On the other hand of these excellent features, the organic EL display has a problem that an image quality changes over time. Namely, it is known that, when a large current continues to flow in organic EL elements to obtain high luminance, a boundary between an organic material layer and electrodes composing an organic EL element is deteriorated due to heating and quality of the organic material layer itself declines over a long period of use time.
To improve the characteristic deterioration of organic EL elements, an improvement in terms of materials of an organic luminous layer and an electrode layer, etc. has been pursued.
On the other hand, to extend the life of self-luminous pixels using organic EL elements, techniques of automatically adjusting luminance are known.
Among them, as a technique of extending the life of light emitting elements by preventing an excessive current from flowing to the light emitting elements, for example, there is known a drive control technique of a panel for detecting a current flowing to light emitting elements by a voltage supply line shared by a plurality of light emitting elements and optimizing luminance of an image based on the detection result (for example, refer to the patent article 1: the Japanese Unexamined Patent Publication No. 2002-215094, pp. 4 to 6, first and second embodiments and FIG. 1 to FIG. 3). In the patent article 1, two methods are disclosed as a control method of light emitting luminance of organic EL elements.
The first method is to make a drive voltage to be applied to organic EL elements connected in series with a TFT transistor and a TFT transistor driven by a horizontal scan line variable, and optimize the drive voltage based on a detection result of the current explained above.
The second method is to change a duty ratio of a light emission time based on the detection result of the current explained above, that is, a pulse width of a signal to control a light emission time.
It is known that light emitting materials of red (R), green (G) and blue (B) used in respective pixels in a screen display region of an organic EL panel differ between colors, and deterioration characteristics over time along with light emission also differ between colors. In this case, color balance changes from an initial stage of image display to a stage after a certain time, so that some image quality (color balance) adjustment mechanism becomes necessary to maintain a high image quality for a long time (for example, 10 years). Also, due to production fluctuation of panels, color balance of products is different from a set value, so that a color balance adjustment mechanism becomes also necessary for that.
However, when applying the first method and the second method described in the above patent article 1 for color balance adjustment, a drive voltage controller illustrated in FIG. 1 or a duty ratio controller illustrated in FIG. 2 in the patent article 1 is necessary for each color. Thus, there is a first problem that a color balance adjustment circuit becomes large in scale and chip cost rises. In the above patent article 1, a specific method of adjustment for each color is not disclosed.
Also, particularly in the second method, that is, a method of changing the duty ratio of a signal for controlling a light emission time, there is an advantage in that deterioration of the light emitting element characteristic is hard to accelerate comparing with that in the first method and power consumption is suppressed because the drive voltage level of the organic EL elements is set to be constant, but quality of the displayed image is affected depending on a drive frequency of the display panel. Namely, in the case where vertical and horizontal drive frequencies are high on a wide screen having a large number of pixels, flickering impression called a flicker on the screen is increased in some cases when the light emitting time is made short. Also, particularly in the case of a motion picture, when the light emitting time is made longer, it looks like an image blurs at a moment of switching screen between fields or frames. Namely, when a light emission time is long, an organic EL panel performs image display close to that on a hold type display, such as an LCD display for emitting light over one horizontal period, and motion picture characteristics are declined. Accordingly, since a light emission time of pixels has an optimal range for an operation frequency in an organic EL display, control of that is limited only with the second method of controlling the light emission time, which is a second problem.